1. Field of the Invention
The present invention relates to a semiconductor device having, for example, a current mirror circuit, a photoelectric conversion device, and an image reading apparatus and, more particularly, to a photoelectric conversion device having, e.g., a CMOS current mirror constant current source circuit, and an image reading apparatus having a multisensor in which a plurality of photoelectric conversion devices are arranged.
2. Description of the Related Art
In recent years, in the field of photoelectric conversion devices, one in which a light-receiving element and its peripheral circuit are formed on a single substrate has be-en extensively developed.
Examples of such a photoelectric conversion device are a linear sensor in which an operational amplifier and light-receiving element are formed on a single semiconductor substrate (Journal of Television Society, Vol. 47, No. 9 (1993), p. 1,180), an image sensor having a sample-and-hold circuit (Japanese Patent Laid-Open No. 4-223771), and a solid-state image sensor having an internal reference voltage generation circuit constituted by an operational amplifier (Japanese Patent Laid-Open No.9-65215).
A bias current for an operational amplifier is generally generated using a constant current source circuit. When this constant current source circuit is formed using a MOS transistor, a CMOS constant current source circuit (R. Gregorian and G. C. Temes, Analog MOS Integrated Circuits for Signal Processing, p. 127, FIGS. 4 and 5) like the one shown in FIG. 10 is generally employed. In addition, a CMOS constant current source circuit like the one disclosed in Japanese Patent Laid-Open No. 7-44254 is proposed.
Operation of a conventional CMOS constant current circuit will be described with reference to FIG. 10.
FIG. 10 shows the state in which power supply voltage is applied to the conventional CMOS constant current circuit that constitutes a current mirror circuit. Referring to FIG. 10, a current flowing from the drain of Q3 to the drain of Q2 is equal to a constant current flowing from the drain of Q4 to the drain of Q1. In general, the constant current circuit stabilizes while the constant current flows (the MOS transistors Q2 and Q4 operate in a saturation region in FIG. 10).
FIG. 11 is a plan view showing the pattern of the conventional constant current circuit (FIG. 10). FIG. 12 is a schematic sectional view taken along the line A-A' in FIG. 11. The NMOSs Q1 and Q2 and PMOSs Q3 and Q4 in FIG. 10 correspond to NMOSs 104 and 103 and PMOSs 101 and 102 in FIG. 11, respectively.
As is apparent from FIGS. 11 and 12, the conventional PMOS and NMOS have substantially the same opening area of the drain region.
However, in a photoelectric conversion device in which the CMOS constant current circuit disclosed in the prior art and a light-receiving element are formed on a single semiconductor substrate, the CMOS constant current circuit may fail to operate upon irradiation of light. More specifically, the CMOS constant current circuit may stabilize while almost no constant current flows (V.sub.01 .apprxeq.V.sub.DD, V.sub.02 .apprxeq.GND in FIG. 10). In this state, since V.sub.01 and V.sub.DD have substantially no potential difference, almost no bias current flows, so the circuit does not operate normally.
This reason will be described.
For example, when photocarriers are generated at a p-n junction formed by the drain (P type) of the PMOS transistor Q3 and a substrate (well: N type), emission holes are accumulated in V.sub.01 in FIG. 10. As a result, the potential of V.sub.01 rises to turn off the PMOS transistors Q3 and Q4. Accordingly, the potential of V.sub.02 falls, and the constant current circuit finally stabilizes while almost no constant current flows.
Also when photocarriers are generated at a p-n junction formed by the drain (N type) of the NMOS transistor Q1 and a well (P type), emission electrons generated in Q1 are accumulated in V.sub.02, and thus the potential of V.sub.02 falls. The constant current circuit stabilizes while almost no constant current flows, and fails to operate normally. Hence, when the constant current circuit cannot be satisfactorily shielded from light under limitations on the pattern layout and design rule of a semiconductor substrate, the constant current circuit fails to operate normally.